Motor control for skip hoist drive systems and the like

ABSTRACT

Described is an electrical motor control for skip cars of the type utilized to charge materials into a blast furnace, and for other similar applications, wherein a first current or torque limit of the motor is provided under decelerating conditions and a second and different current or torque limit is provided under accelerating conditions. This tends to equalize the pulling forces in the cables which elevate the skip cars under deceleration and under acceleration conditions and reduces strains in the skip bridge and associated structural elements of the blast furnace.

United States Patent Inventor William A. Munson [72] 2,753,504 7/1956 Pell 318/434 Williamsville, N.Y. 3,305,720 2/1967 Safar 318/434 [21] Appl. No. 756,652 3,477,006 11/1969 Fair etal 318/302 [22] Filed Aug. 30,1968 3,108,214 10/1963 Wilkerson 318/158 [45] Patented June 29, 1971 3,378,749 4/1968 Cunningham 318/144 [73] Assignee Westinghouse Electric Corporation 3,407,904 10/1968 Trollope 318/144 Pittsburgh, Pa. 3,458,790 7/1969 Wilkerson 318/258 Primary Examiner-Cris L. Rader s4 MOTOR CONTROL FOR SKIP HOIST DRIVE 43mm EmmiwhK' SYSTEMS AND THE LIKE Attorneys-F. H. Henson, R. G. Brodahl and C. J. Paznokas 7 Claims, 3 Drawing Figs. [52] US. Cl 318/144,

318/58 318/269 318/302 318/434 ABSTRACT: Described is an electrical motor control for skip ll?- cars f th t tilized to charge materials into a blast fur. [50] Field of Search 318/144, ace, and f other similar applications wherein a fi t current 146, 158, 258, 269,376, 302,434,430, 4 or torque limit of the motor is provided under decelerating 56 R i cud conditions and a second and different current or torque limit 1 e "antes I is provided under accelerating conditions. This tends to equal- UNITED STATES PATENTS ize the pulling forces in the cables which elevate the skip cars 3,508,132 4/1970 Peterson 318/271 under deceleration and under acceleration conditions and 2,379,958 7/1945 Fox 318/308 reduces strains in the skip bridge and associated structural ele- 2,519,212 8/1950 Allbert et a1. 318/434 ments of the blast furnace.

I2 I I6 ,-22

L l VOLTAGE 1 SENSOR l 72 74 I82 90 VOLTAGE CURRENT THYRISTOR CONTROLLER CONTROLLER cfi a dt i l r 70 I l 1 l 1 I as l. 78 CURRENT CURRENT TRANSDUCER LIMITER k} ACC. 92 BO-QT g4 cu RENT ug T ER T SHEET 1 IF 2 FIG] 66 VOLTAGE K/ SENSOR v N MOTOR 9 FIELD 72 74 82 90 CONTROL VOLTAGE CURRENT THYRISTOR E A CONTROLLER -'C0NTR0LLER fi' 'iffi 70 A 62 64 88 86 1 CURRENT CURRENT TRANSDUCER LIMITER l ACC. 92

CURRENT LIMITER DEC.

WITNESSES INVENTOR HM, %.Ma W|l||c|m A. Munson I ATTORNEY PATENIEIIIIIIIzsIsiI 3,590,350

sum 2 0F 2 VOLTAGE SENSOR VOLTAGE CURRENT THYRISTOR Q CONTROLLER CONTROLLER TS ID Q qIo I cu sI t IIIr I08 AGE 88 62 I 1 I A 80 CURRENT U LIMIT DEC.

- CURRENT TRANSDUCER g 3 FIG. 2

, 'I SO FIG. 3

CURRENT LIMIT (MOTORING) I I I III/I I II QQI JII III" I I I (REGENERATIVE) I I I I I I l I I I .I I l I I I I I I I I I I I II *2 *3 '4 s '7 TIME CROSS-REFERENCES TO RELATED APPLICATIONS None.

BACKGROUND OF THE INVENTION While not limited thereto, the present invention is particularly adapted for use as a motor control for a blast furnace skip car system. As is known, ore, coke and limestone are charged into the top of a blast furnace by means of a pair of skip cars which run on adjacent inclined tracks (usually called a skip bridge) leading from ground level to the top of the furnace. These skip cars are interconnected by means of a cable and winch system such that as filled car ascends the tracks, the other, empty, car descends.

As a filled car starts it ascent, a very large torque is required to accelerate it to the desired speed. Similarly, when the filled car reaches the top of the skip bridge, a reverse torque is required with the drive motor operating under regeneration conditions. When accelerating, the torque must be sufficient to overcome the inherent weight of the skip car as well as the friction of the system. However, when decelerating, the friction of the system assists in slowing down the car.

If the same current limit is used on the drive motors for both accelerating and decelerating conditions, it must be high enough to develop a torque sufficient for startup and acceleration; but this current limit causes excessive torque when decelerating, resulting in variations in pull on the cables of 2 to l or better. This creates excessive stresses in the structural members of the skip bridge and blast furnace structure and materially reduces the life of the cable or cables which pull the skip cars.

SUMMARY OF THE INVENTION As an overall object, the present invention provides a means for reducing stresses on a skip car drive system of the type described above due to excessive torque during deceleration.

More specifically, an object of the invention is to provide a motor control system for skip car drive systems and the like wherein a first current limit value is used for accelerating (motoring) conditions and a second, lower, current limit value is used for decelerating (regenerating) conditions.

In accordance with the invention, a motor control system for skip car drives and the like is provided including a voltage reference controller, a current controller responsive to current through the motor coupled to the output of the voltage reference controller, and means coupled to the output of the current controller for varying the voltage and current supplied to the motor.

Two current limit circuits are provided, the first of such circuits being responsive to current through the motor under motoring conditions only, and the second current limit circuit being responsive to current through the motor only under regeneration conditions. The output of the first current limit circuit is utilized to vary the maximum output of the voltage reference controller under motoring conditions, while the output of the second current limit circuit is utilized to vary the maximum output of the voltage reference controller under regeneration conditions.

Actual test conditions show that where 300 percent current limit may be required to accelerate the skip cars, a current limit of only 75 percent can be used for deceleration purposes, resulting in a reduction of cable pull to that approaching the pull required during acceleration. Thus, by setting the current limit value of the second current limit circuit much lower than that of the first, the strains imposed on the mechanical system can be reduced and the expected cable life of the hoist itself can be increased,

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic illustration of one embodiment of the invention as applied to a thyristor-powered motor drive for a skip hoist system;

FIG. 2 is a schematic illustration of another embodiment of the invention as applied to a motor-generator drive system for the skip hoist; and

FIG. 3 comprises waveforms illustrating the operation of the systems ofFIGS. 1 and 2.

With reference now to the drawings, and particularly to FIG. 1, the reference numeral 10 designates the receiving hopper of a blast furnace, schematically illustrated at 12. Extending upwardly from ground level to the top of the blast furnace 12 is a skip bridge 14 comprising a pair of side-by-side tracks 16 on which skip cars 18 and 20, respectively, move. The two skip cars 18 and 20 are interconnected by means of a cable 22 which extends from skip car 20 around sheaves 24 and 26 and then to a winch drum 28 around which it is wound a number of times. From winch drum 28, the cable 22 passes around sheaves 30 and 32 and then to skip car 18.

Upon rotation of the drum 28 in the direction of arrow 34, for example, the skip car 20 will be caused to move up its tracks, while the skip car 18 is permitted to move downwardly on the adjacent track. At the bottom of the skip bridge 14, materials to be charged into the blast furnace are fed into skip car 20, for example, from chute 36. When the skip car reaches the top of the skip bridge 14, its rear wheels are caused to move along track section 38 and its forward wheels along curved track section 40, thereby tipping the skip car to discharge its contents into the receiving hopper 10 of the blast furnace 12.

As was explained above, the total torque on the winch drum 28 during accelerating conditions is that due to the load itself as well as the friction of the system. This requires large current surges in the drive motors for the winch drum 28 when the skip car is initially lifted off ground level and after it passes the lower, curved track section and accelerates upwardly along the straight track sections of the skip bridge 14. On the other hand, when the skip car reaches the top of the skip bridge and must be decelerated, the friction of the system assists in the deceleration process and, thus, the reverse torque required on the winch drum 28 is materially reduced.

The foregoing can perhaps best be explained by reference to waveforms A and B of FIG. 3. In waveform A, the solid curve 42 represents the input speed control signal for motor rotating the winch drum 28. At time t,, the motor is turned on to operate at a relatively low speed until the skip car clears the lower, curved portion of the tracks 16. However, the actual speed of the skip car, represented by the broken line 44 in FIG. 3, does not reach the desired speed immediately; and during this time a large current surge, represented by the pulse 46 in waveform B, is required by the motor. After the skip car 20 clears the lower, curved section of tracks 16, its input speed signal increases abruptly at time 2 however the actual speed of the motor and skip car again increases gradually while the current input to the motor must increase to the maximum current limit value as illustrated by pulse 49 in waveform B. From time t to time t;,, the skip car travels up the skip bridge 14 at a relatively constant speed while the current input to the motor remains constant. At time I however, the speed signal is decreased abruptly, whereupon the skip car speed decreases under decelerating conditions as illustrated by the broken line 50. Under these circumstances, the motor driving the winch drum 28 are operating under regenerative conditions. Hence, a negative current surge represented by pulse 52 in waveform B is produced. Finally, at time the speed signal drops to zero, whereupon the skip car also decelerates to zero along the line 54, producing a second negative current surge represented by the pulse 56 in waveform B. Between times and t the skip cars 18 and 20 are stopped, and at time t the process is repeated with the motor rotating in the opposite direction.

In accordance with the present invention, the current limit under regenerative conditions, indicated by line 58 in FIG. 3, is much lower than the current limit under motoring conditions, represented by the line 60. This, then, reduces the torque imposed on the system by the motors during decelerating conditions and tends to eliminate the stresses in'the drive cables and structural parts which occur when the decelerating torque is higher.

One system for achieving the operational characteristics illustrated by the waveforms of FIG. 3 is shown in FIG. 1 wherein the drive motor for the winch drum 28 is schematically represented by a single motor 62 driven by a thyristor power supply. It should be understood that while only one motor 62 is shown herein, an actual drive may include two or more motors connected to the drum 28 through a gear train. In this latter case, all motors are controlled simultaneously in the same manner as the single motor shown herein. Motor 62 is connected to a source of alternating current supply voltage 64 through a dual converter, schematically illustrated by thyristor elements 66 and 68. The thyristor element 66, in the usual three-phase alternating current supply system, represents six separate thyristors; and similarly, the thyristor element 68 represents six separate thyristors.

An input control signal for the motor 62, such as that represented by the voltage level trace 42 of wave form A in FIG. 3, is applied to input terminal 70 and thence to a voltage controller 72 having a gain curve represented by the curve 74. As can be seen, the gain of the circuit gradually increases in the positive or negative direction, depending upon the polarity of the input signal, until it reaches a point of saturation. Furthermore, the saturation point can be varied upwardly or downwardly as shown, for example, by the dotted .line on the gain curve 74. v

The inputs to the speed controller 72 are both the input control voltage on terminal 70 and a feedback voltage from voltage sensor 76 representing motor speed. The voltage sensed by voltage sensor 76 is proportional to armature voltage and, hence, speed. The sensor 76, however, could be replaced by a tachometer generator connected to the motor shaft with the same overall effect. When the voltage on terminal 70 and that from voltage sensor 76 do not cancel, meaning that the actual speed of the motor is not equal to that dictated by the speed control signal on terminal 70, the voltage controller 72 integrates the difference and the output will increase until its saturation limit is reached in either the positive or negative direction. Once the saturation limit is reached in either the positive or negative direction, the speed of the motor cannot increase further. Assuming that the saturation limit (i.e., speed limit) is not reached, the speed of the motor will increase or decrease until the feedback voltage from sensor 76 matches that on terminal 70, meaning that the motor is rotating at the desired speed as determined by the magnitude of the signal on lead 70. The speed controller saturation limits, in turn, are set by one of two current limiter circuits 78 or 80. These circuits act to limit the maximum speed of the motor as a function of current through the motor armature. As will be seen, the current limiter circuit 78 is effective during accelerating conditions while circuit 80 is effective during deceleration conditions. Furthermore, the limiting value of circuit 78 is much higher than that of circuit 80, whereby greater surges of current can occur under accelerating conditions than under decelerating conditions.

The output from the speed controller 72 is one input into a current controller 82. The other input to the current controller on lead 84 is a feedback current from a current transducer 86 which, in turn, is connected across a resistor 88 in the armature circuit for motor 62. When the two inputs to the current controller do not cancel each other, the current controller integrates the difference and its output changes the firing angles of thyristors 66 and 68 via thyristor firing circuit 90, thereby causing the voltage to the motor and, hence, its speed to change.

As can be seen by reference to waveform B of FIG. 3, the motor 62 is operating in the forward direction between times t, and t and in the reverse direction between tines t and r, where the polarity of the reference input signal is reversed. The thyristor power supply is such that current through the motor armature 62 does not reverse when changing from the forward to reverse direction. Rather, it, reverses only under regeneration conditions. Reversal of the motor is achieved by reversing the current through its field winding 91 via motor field control circuit 93.

Since the current through the motor armature does not reverse except under regeneration conditions for both the forward and reverse directions of rotation, the output of the current transducer-can be applied through diode 92 to the accelerating current limiter circuit 78 and through diode 94 to the decelerating current limiter circuit 80. Hence, positive current surges will actuate circuit 78 only and, similarly, negative current surges will actuate circuit only. As was mentioned above, the limiting value of circuit 80 is set much lower than that of circuit 78 wherebyv the reduced current limit effect is achieved under decelerating conditions.

FIG. 2 illustrates a motor drive system for the skip cars wherein the current through the motor armature reverses when going from forward to reverse rotation or vice versa. The circuit of FIG. 2 is similar to that of FIG. 1 and, accordingly, elements of FIG. 2 which correspond to those of FIG. 1 are identified by like referencenumerals. In this case, again, the voltage feedback signal to the voltage controller 72 is derived from a voltage sensor 76 representing motor speed. Furthermore, the motor 62 is not driven by a dual converter but rather by a generator 98 in a Ward-Leonard system wherein the generator 98 is driven by means of a conventional three-phase motor 100. The field winding 102 for the generator 98 is controlled by a thyristor power supply 104 which is, in turn, responsive to the output of the current controller 82. By varying the field of generator 98, the voltage and current to motor 62 can be varied. In this type of arrangement, the current through the motor armature 62 is reversed as shown by waveform C in FIG. 3. As a result, the diode system of FIG. I will not sufiice. Rather, a relay system must be employed. This includes relay 106 having normally closed contacts 108 connecting transducer 86 to the accelerating current limiter 78 and normally open contacts 110 adapted to connect the transducer 86 to decelerating current limiter 80. The coil of relay 106, in turn, is actuated by a suitable type of limit switch apparatus, schematically illustrated at 112, which will energize the relay and reverse its contacts as either one of the skip cars approach the top of the skip bridge prior to deceleration. In this manner, the decelerating current limit is switched into the system just prior to the start of deceleration.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In combination, a drive system comprising electrical drive motor means having armature means, speed control means for varying the voltage applied to said drive motor means to thereby vary the speed of said drive motor means, means for sensing the actual current flowing through said armature means, first means operative only under motoring conditions and connected to said sensing means for limiting current flow through said motor means and the output torque of said motor means to a first predetermined maximum value, and second means operative only under regenerating conditions and connected to said sensing means for limiting current flow through said motor means and the output torque of said motor means to a second predetermined maximum value which is lower than said first predetermined maximum valve.

2. The combination of claim 1 including a voltage reference controller, a current controller responsive to current through said motor coupled to the output of said voltage reference controller, and means coupled to the output of said current controller for varying the voltage and current supplied to said motor means.

3. The combination of claim 2 wherein the means coupled to the output of said current controller comprises a thyristor power supply.

4. The combination of claim 2 wherein the means coupled to the output of said current controller comprises the field winding of a direct current generator, the output of said direct current generator being connected to the input of said drive motor means.

5. The combination of claim 2, with said first means including a first current limit circuit responsive to current through the motor means under motoring conditions, with said second means including a second current limit circuit responsive to current through the motor means under regenerating conditions, and means connecting the outputs of said first and second current limit circuits to said voltage reference controller to regulate the maximum output of said voltage reference controller.

6. The combination of claim 1 in further combination with a blast furnace charging system comprising a skip bridge, a pair of skip cars reciprocable on said skip bridge, cable means interconnecting said skip cars, a rotatable winch drum around which said cable is wound whereby rotation of the winch drum will cause one of said skip cars to ascend the skip bridge while the other descends, and means connecting said drive motor means to said winch drum whereby the current through said motor means may be higher during acceleration conditions when a loaded skip car initially ascends the skip bridge than when a skip car decelerates at the top of the skip bridge, thereby minimizing vibrational shocks to said skip bridge as a result of sudden deceleration of the skip cars.

7. The combination of claim 6 wherein said first means includes a first current limiting circuit responsive to current through the motor means when said skip cars accelerate, and wherein said second means includes a second current limiting circuit responsive to current through the motor means when said skip cars decelerate, and means connecting the outputs of said first and second current limiting circuits to said means for varying the voltage applied to said drive motor means. 

1. In combination, a drive system comprising electrical drive motor means having armature means, speed control means for varying the voltage applied to said drive motor means to thereby vary the speed of said drive motor means, means for sensing the actual current flowing through said armature means, first means operative only under motoring conditions and connected to said sensing means for limiting current flow through said motor means and the output torque of said motor means to a first predetermined maximum value, and second means operative only under regenerating conditions and connected to said sensing means for limiting current flow through said motor means and the output torque of said motor means to a second predetermined maximum value which is lower than said first predetermined maximum valve.
 2. The combination of claim 1 including a voltage reference controller, a current controller responsive to current through said motor coupled to the output of said voltage reference controller, and means coupled to the output of said current controller for varying the voltage and current supplied to said motor means.
 3. The combination of claim 2 wherein the means coupled to the output of said current controller comprises a thyristor power supply.
 4. The combination of claim 2 wherein the means coupled to the output of said current controller comprises the field winding of a direct current generator, the output of said direct current generator being connected to the input of said drive motor means.
 5. The combination of claim 2, with said first means including a first current limit circuit responsive to current through the motor means under motoring conditions, with said second means including a second current limit circuit responsive to current through the motor means under regenerating conditions, and means connecting the outputs of said first and second current limit circuits to said voltage reference controller to regulate the maximum output of said voltage reference controller.
 6. The combination of claim 1 in further combination with a blast furnace charging system comprising a skip bridge, a pair of skip cars reciprocable on said skip bridge, cable means interconnecting said skip cars, a rotatable winch drum around which said cable is wound whereby rotation of the winch drum will cause one of said skip cars to ascend the skip bridge while the other descends, and means connecting said drive motor means to said winch drum whereby the current through said motor means may be higher during acceleration conditions when a loaded skip car initially ascends the skip bridge than when a skip car decelerates at the top of the skip bridge, thereby minimizing vibrational shocks to said skip bridge as a result of sudden deceleration of the skip cars.
 7. The combination of claim 6 wherein said first means includes a first current limiting cirCuit responsive to current through the motor means when said skip cars accelerate, and wherein said second means includes a second current limiting circuit responsive to current through the motor means when said skip cars decelerate, and means connecting the outputs of said first and second current limiting circuits to said means for varying the voltage applied to said drive motor means. 